This invention relates to a method for manufacturing a combination cover for closing the chip cavity of a semiconductor package and, in particular, to manufacturing a corrosion proof combination cover for closing and sealing a high reliability micro electronic package.
In the packaging of high reliability micro-circuits, a semiconductor chip is typically housed with a cavity formed in a ceramic housing and a lid is sealed to the housing to close the package. The lid is generally formed of an iron based metal called Kovar and the seal is formed by a pre-formed solder frame that is placed between the lid and package housing. In U.S. Pat. No. 3,946,190 there is described what is generally referred to as a combination cover wherein the solder frame is tack welded to the Kovar lid. The combination cover is capable of being hermetically sealed to the package to protect the enclosed chip.
The Kovar lid of a high reliability package is typically electroplated with a base coating of nickel and a final coating of pure gold. The nickel layer is usually about 100 micro inches thick while the gold outer layer is about 50 micro inches thick. The gold overlayer provides an oxide free surface for the solder which insures that the solder will uniformly wet the lid and thus produce the desired hermetic seal. It should be noted, however, that both coating layers are crystalline in form and thus contain cracks at the grain boundaries along which corrosive atmospheres can attack the iron base Kovar substrate. When this occurs the lid will rust and the protective coating will spald or flake away further promoting and accelerating the rusting process. This leads to premature failure of the package and degradation of the housed device.
Many high reliability packages are now expected to meet the requirements set out in the military specifications Mil-Std-883 C, Method 1009, Test Condition A, which is a salt atmosphere test. Under this test, the closure lids are exposed to a salt spray atmosphere under closely controlled climatic conditions. After twenty four hours of exposure to the corrosive atmosphere, the lids are washed and rinsed thoroughly and then inspected under a microscope to determine if the part has been adversely affected. A nickel-gold plated Kovar lid will invariably fail test test.
In an effort to adapt the Kovar lid to meet military specifications, a multi-coated structure has been devised wherein four coating layers rather than two are electroplated over the Kovar substrate. As described in U.S. Pat. No. 4,601,958, the multi-layers comprise a base layer of nickel followed by successive layers of gold, nickel and gold. To comply with the military specifications, the final layer of gold must be at least 50 micro inches thick. The four layers, when plated to the suggested thicknesses, function to block or dead end the grain boundaries and thus effectively shield the Kovar substrate sufficiently so that it will not be adversely affected by the salt spray test.
Although the multi-layered lid described above has to some extent solved a problem in the art, the lid has certain disadvantages that the present invention overcomes. A relatively large amount of gold must be used in the coating process which is objectionable from a cost standpoint. Furthermore, the first or underlayer of gold cannot be economically recovered in the event a part has to be scrapped and, as a consequence, scrap losses are high. The multiple coatings are subjected to unwanted stressing as the lid progresses through the various plating baths and stress failures can occur if the bath conditions are not closely controlled. Each of the plating steps needed to produce a coating layer is also time consuming and thus relatively expensive. Finally, even with the use of multiple coating layers, there always exists the possibility that the coating shield can be penetrated and the lid attacked by a corrosive atmosphere.